Scientists including lead author Clement Suavet of the Massachusetts Institute of Technology, examined two mare basalt rocks collected by Neil Armstrong and Buzz Aldrin from the Sea of Tranquillity during the historic first manned mission to the lunar surface in 1969.

They found both samples were magnetised in a stable and surprisingly intense magnetic field of at least 13 micro Tesla.

"This data indicates the magnetic field was continuously active well after the final large basin-forming impacts," they write.

Dynamo dynamics

A global magnetic field is generated by a moving liquid core called a dynamo.

Although the Moon does not have a global magnetic field today, computer studies indicate it had an active core until 4.1 billion years ago, while previous rock sample studies indicate a dynamo was still active 3.7 billion years ago.

There are a number of theories about what generated past lunar magnetism.

One theory is the possibility that it may have been powered by the movement of molten rocks in the core as different minerals crystallised at different temperatures and pressures during cooling -- a process known as a compositional convection dynamo.

Other ideas include tidal heating caused by either changes in the Moon's rotation due to large asteroid impact events, or slight wobbles in the Moon's spin, a process called precession.

"Large impacts have the potential to unlock the Moon from synchronous rotation [with Earth], such that the resulting differential motion between the librating mantle and core could generate a dynamo," they write.

However, when Suavent and colleagues searched for evidence of asteroids large enough to change the Moon's rotation they couldn't find any craters of the right size younger than around 3.8 billion years.

"This likely excludes impact-driven changes in rotation rate as the source of the dynamo at this time in lunar history," they write.

"Rather, our results require a persistent power source like precession of the lunar mantle or a compositional convection dynamo."